Many patent publications, such as WO 96/11300 and U.S. Pat. No. 4,461,095, teach the drying of a fiber web between two parallel metal bands moving in the same direction such that the fiber web is in contact with the heated metal band and that there is a wire between the fiber web and the second, cooled metal band, whereby the steam separated from the fiber web by heating condenses to the wire by the effect of the cold metal band. The basic idea is that two endless metal bands are arranged to run around turning rolls and that against the inner surface defined by the loops that the bands form are provided pressure chambers containing hot steam and water, respectively, such that the pressure produced presses the hot and cold bands, respectively, against the fiber web and the felt running between them. Together with seals, the bands located between the pressure chambers form one side of the pressure chambers such that the steam and water can directly affect the bands. The operation of the apparatus is fully known per se and has been disclosed, for example, in the above patent publications, which are incorporated herein by reference.
A piece of information needed in the above-described control and adjustment of the drying apparatus is the position of the bands crosswise of the machine direction. The position of the edge of the band is measured using a sensor that forms a fork, inside which the edge of the band can move. In both branches of the fork there are optical fibers that form a kind of light nozzle. The light travels from the light-transmitting fiber ends through the air to the light-receiving fiber ends, unless there is a band edge preventing the light from travelling. The edge of the band partly covers the light nozzle, whereby the output signal of the sensor is proportional to the area covered by the band. In the fiber web drying apparatus environment, impurities, however, may cover the light nozzle and the ends of the fibers, and this may change the readings of the sensor, i.e., the sensor will no longer be reliable: it is prone to malfunction and requires much maintenance. The vertical motion of the band is also rather restricted, since the band damages the fibers when it comes into contact with them. Further, the sensor is difficult to adjust, and its readings are not reliable. The settings of the sensor are not stable, which is another reason why the sensor requires much calibration and maintenance. Further, the sensor is rather expensive.